1. Field of the Invention
The invention relates generally to holographic data storage systems, and more particularly to methods and systems for determining misalignments and positioning components in a holographic data storage system.
2. Description of the Related Art
Holographic data storage systems store information or data based on the concept of a signal beam interfering with a reference beam at a holographic storage medium. The interference of the signal beam and the reference beam creates a holographic representation, i.e., a hologram, of data elements as a pattern of varying refractive index and/or absorption imprinted in a volume of a storage or recording medium such as a photopolymer or photorefractive crystal. Combining a data-encoded signal beam, referred to as an object beam, with a reference beam can create the interference pattern at the storage medium. A spatial light modulator (SLM), for example, can create the data-encoded signal beam. The interference pattern induces material alterations in the storage medium that generate the hologram. The formation of the hologram in the storage medium is a function of the relative amplitudes and polarization states of, and phase differences between, the signal beam and the reference beam. The hologram is also dependent on the wavelengths and angles at which the signal beam and the reference beam are projected into the storage medium. After a hologram is created in the storage medium, projecting the reference beam into the storage medium interacts and reconstructs the original data-encoded signal beam. The reconstructed signal beam may be detected by using a detector, such as CMOS photo-detector array or the like. The recovered data may then be decoded by the photo-detector array into the original encoded data.
In typical holographic data storage systems it is important to align the SLM, detector (i.e., a camera), and the data storage medium such that each pixel of the SLM is projected onto a single pixel of the detector. This alignment is desired for a single hologram or a group of holograms stored by various multiplexing methods including angle, shift, wavelength, correlation, spatial, aperture, phase code, and the like. It is further important to align holograms recorded in holographic medium such that, during hologram readout, each bit in the hologram image is projected onto a single pixel of the detector. Aligning the pixels of the SLM, stored holographic image, and detector is commonly referred to as “pixel matching.” The objective of pixel matching is to obtain recovered images of data-containing holograms on the detector that have a low number of bits decoded in error in relation to the total bits of the data page, i.e., a low bit error rate (BER).
Performance of a holographic storage system, i.e., the quality of the modulated image, therefore depends at least in part on the alignment of various components such as the SLM with various devices, such as light sources, lenses, detectors, the storage medium, and the like. Generally the position and alignment of the SLM and other device components for reading and writing to the storage medium are mechanically set at the time of manufacturing the holographic storage system. Over time, however, the SLM, detector, or storage medium may become misaligned with various other components of the particular system. For example, temperature change, vibration, shock, and the like may result in slight movements of the detector, SLM, storage medium, or other optical components. The result may be translational, tilt, or rotational misalignment of the detector with the medium or the SLM. Further, in systems with removable storage medium, such as a rotating disk or rectangular medium, the medium may be misaligned when inserted into the system.
One strategy proposed for correcting errors arising from pixel misalignment due to improperly positioned system components is to apply image-processing techniques to the recovered hologram image. An example of such a method is described in “Compensation for Pixel Misregistration in Volume Holographic Data Storage,” by G. W. Burr and T. Weiss published in Optics Letters, Vol. 26, No. 8, Apr. 15, 2001, the entire content of which is incorporated herein by reference. Another approach is to adjust or tilt the reference beam to realign the image onto the detector during a read out process. Such a method is described in U.S. Pat. No. 5,982,513 entitled “Method and System to Align Holographic Images,” the entire content of which is incorporated herein by reference. Neither of the above approaches reposition components of the system and therefore have limited ability to compensate for alignment errors.
New methods and devices are therefore needed for reliably measuring and adjusting for pixel misalignments. In particular, methods and systems for moving the components of a holographic storage system, including the SLM, detector, and/or storage medium are needed. Further, methods and systems for determining the direction and magnitude of a misalignment of detected pixels, and the type of misalignment of a device component is needed.
The present invention satisfies these needs.